1. Field of the Invention
The present invention relates to processing of fermented feedstock and, more particularly, a method and apparatus for producing bioethanol, concentrated stillage, and pure water.
2. Discussion of the Related Art
Ethanol (ethyl alcohol) is commonly used in the United States, Brazil, and other countries as a biofuel additive for gasoline and for other purposes. In the United States, for example, motor fuel or gasoline for automobiles will often contain up to ten percent ethanol. In Brazil, the concentration of ethanol is typically from two percent to ninety percent. A small percentage of global production of ethanol is derived, from petroleum, with the overwhelming majority of ethanol being produced from feedstock, such as corn, sugar cane, sugar beet, sorghum, switchgrass, barley, potatoes, sweet potatoes, fruit, molasses, grain, and wheat. Ethanol that is produced from feedstock is commonly referred to as “bioethanol.”
Bioethanol is produced from fermented feedstock and, more particularly, from microbial fermentation of sugars contained in the feedstock. In the case of corn-based bioethanol, the corn is first “mashed” to break the corn starch down to sugars before being exposed to the microbial fermentation process. Mashing is not required for sugar cane, sweet sorghum and other feedstock that are high in natural sugar content. In either event, the fermented material is then distilled to remove its water content, thus providing essentially water-free (near 200 proof) ethanol that can be used, e.g., as a gasoline additive. Specifically, the fermented mixture, sometimes called “beer” or “Vanessa”, is pumped to a distillation system and heated. The differences in the boiling points of ethanol and water (78° C. or 172° F. for ethanol vs. 100° C. or 212° F. for water) are used to boil off and separate the ethanol. The ethanol vapor will typically contain approximately five percent water. To further reduce the water content, the ethanol vapor is condensed and passed through a sieve to yield near 200 proof, waterless ethanol.
In addition to producing ethanol, as described above, one of the byproducts of the distillation process is a heavily watered distillate known generally as “stillage” or “stillbottoms”. The stillage is composed of insolubles, i.e., solids from the feedstock and yeast added for fermentation, and liquid from the water added during the process. The insolubles are generally nutrient rich and can be used for fertilizer, livestock dressing, and the like. Most of these applications require significant dewatering of the initial or “thin” stillage to produce “concentrated stillage” that has a solids content of on the order of 25% to 65% or higher. About 85% of the total stillage volume typically is removed from the stillage during this dewatering. Some applications require essentially complete dewatering and other post-processing of the concentrated stillage.
Stillage dewatering efforts have included mechanical separators, sieves and screens, and more recently, evaporators. As their name suggest, mechanical separators are designed to mechanically separate the insolubles from the water. Numerous separator designs have been developed, but their success has been limited. One of the principle drawbacks of mechanical separators and similar systems that are based on screens and sieves is that copious volumes of water must be pumped through the dewatering system. For example, it is not uncommon for a bioethanol production plant to pump 280,000 or more pounds of thin stillage per hour through its dewatering system. Needless to say, moving that much stillage and water, at that rate, requires a tremendous amount of energy.